Scintillators are substances which, when hit by radiation such as alpha rays, beta rays, gamma rays, X-rays, or neutrons, absorb the radiation to emit fluorescence. The scintillator is combined with a photodetector, such as a photomultiplier tube, and such a combination is used as a radiation detector.
The radiation detector is widely utilized in diverse application fields, including resource exploration fields such as well logging of oil fields, medical fields such as tomography, industrial fields such as nondestructive inspection, security fields such as inspection of personal belongings, and academic fields such as high energy physics.
The radiation detector used for well logging of oil fields is installed within an excavating drill, and is used to detect gamma rays or neutrons under excavation and predict the properties of strata (Patent Document 1). Since the temperatures fluctuate greatly during use under excavation, scintillators used for well logging of oil fields are required to have satisfactory characteristics in a wide range of temperatures of lower than 0° C. to higher than 200° C. (see Non-Patent Document 1)
As shown in FIG. 1 of the Non-Patent Document 1, however, the scintillators generally posed the problem of decreasing in the light yield under high temperature environments. For example, the light yield at about 110° C. from bismuth germanium oxide (BGO) decreases to about 16% as compared with the light yield at room temperature. Similarly, the light yield from cadmium tungsten oxide (CdWO4) is reduced to about 20% at about 150° C. Even thallium-doped sodium iodide (Tl:NaI) and cesium fluoride (CsF), which are considered to have relatively satisfactory light emission characteristics under high temperature environments, were problematical in that their light yield decreased to about 70% and about 61%, respectively, under a high temperature environment at about 140° C.